Dell S4820T, Configuration Manual

Page: 410 / 1178

2. The querier sends a Group-Specific Query to determine whether there are any remaining hosts in
the group. There must be at least one receiver in a group on a subnet for a router to forward
multicast traffic for that group to the subnet.
3. Any remaining hosts respond to the query according to the delay timer mechanism (refer to
Adjusting Query and Response Timers ). If no hosts respond (because there are none remaining in
the group), the querier waits a specified period and sends another query. If it still receives no
response, the querier removes the group from the list associated with forwarding port and stops
forwarding traffic for that group to the subnet.
IGMP Version 3
Conceptually, IGMP version 3 behaves the same as version 2. However, there are differences.
• Version 3 adds the ability to filter by multicast source, which helps multicast routing protocols avoid
forwarding traffic to subnets where there are no interested receivers.
• To enable filtering, routers must keep track of more state information, that is, the list of sources that
must be filtered. An additional query type, the Group-and-Source-Specific Query, keeps track of state
changes, while the Group-Specific and General queries still refresh the existing state.
• Reporting is more efficient and robust: hosts do not suppress query responses (non-suppression
helps track state and enables the immediate-leave and IGMP snooping features), state-change reports
are retransmitted to insure delivery, and a single membership report bundles multiple statements from
a single host, rather than sending an individual packet for each statement.
The version 3 packet structure is different from version 2 to accommodate these protocol
enhancements. Queries are still sent to the all-systems address 224.0.0.1, as shown in the following
illustration, but reports are sent to the all IGMP version 3-capable multicast routers address 244.0.0.22, as
shown in the second illustration.
Figure 43. IGMP Version 3 Packet Structure
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Internet Group Management Protocol (IGMP)